Process for the deresination of pulp and use of carbon dioxide or (bi) carbonate therefor

ABSTRACT

The invention relates to a process for the deresination of pulp as well as to the use of carbon dioxide or a (bi)carbonate compound for the deresination of pulp. The process concerns the removal of wood extractives or resin from an aqueous pulp suspension in an alkaline process stage of the pulping process, such as an oxygen delignification stage, at a pH above 10. Carbonate ions are provided in the pulp at the high pH for solubilizing a significant part of the wood extractives in the pulp. The alkaline pulp is dewatered to remove solubilized extractives and for providing a deresinated pulp.

FIELD OF THE INVENTION

The present invention relates to a process for the deresination of pulpas well as to the use of carbon dioxide or a (bi)carbonate compound forthe deresination of pulp. More specifically, the invention relates tothe removal of wood extractives or resin from an aqueous pulp suspensionin an alkaline process stage of the pulping process.

BACKGROUND OF THE INVENTION

A problem associated with the production of pulp from wood either bychemical, mechanical or semi-mechanical processes is the liberation ofwood extractives from the wood fibres into the aqueous phase of the pulpsuspension. The wood extractives are solvent extractable organicmaterials and they are typically composed of a variety of more or lesswater-insoluble compounds, which form sticky particles and agglomerates(pitch) detrimental to the papermaking process and products. Thecompounds designated herein as wood extractives or resin include mainlyfatty acids, resin acids, terpenes, sterols and phenols. Especially, thefatty and resin acids tend to form insoluble calcium salts, which aredifficult to remove.

In chemical pulping, extractives cause problems by forming deposits onthe equipment and on the fibers. Wires and screens may be clogged anddark specks are formed in the bleached pulp or paper. The problem isaccentuated when chlorine free processes are developed and when the useof hardwood as a raw material increases. Pulp from hardwood containseven more extractives than softwood and this tends to increase theseverity of the problems encountered in the handling of the pulp,especially in the bleaching and in the papermaking processes. Mechanicalpulping also liberates extractives into the aqueous phase and, dependingon the operating conditions, the processing of the pulp is disturbed toa greater or lesser degree by pitch.

In kraft processes using hardwood raw material, one way to alleviate thepitch problem is to add tall oil from softwood cooking into the hardwoodcooking process. Other processes for reducing the pitch problem includeadding soaps or other surfactants to the pulp to remove the detrimentalsubstances. One such method is described in WO 2005/080672 (Kemira),which uses an organic stabilizing compound and a surfactant in aperoxide bleaching stage and then removes the extractives by dewateringor washing. WO 2004/101882 (Ciba) describes the use of water-solublecationic polymers for controlling pitch deposits. CA 1162358 (MoDo) usesa mild mechanical working in a special screw to deresinate a cellulosepulp. EP 1 205 598 (SCA) uses supercritical carbon dioxide forextracting wood extractives from dry pulp. The extract is then trappedin an organic solvent.

The book “Pitch control, Wood Resin and Deresination” edited by Ernst L.Back, et al., TAPPI Press Technology Park/Atlanta, 2000, pp 212-218, thecontents of which are considered to be included by reference in thepresent specification, notes (p. 212) that the pH and the content ofmultivalent ions such as Ca and Mg in the washing water influences theresin content of pulps after the cooking. It is noted (p. 216) that theformation of calcium soaps late in the pulp washing operation may causeproblems for deresination. However, these problems may be alleviated byblocking the free calcium by carbonate ions in the late stage ofwashing, or the washing may be improved by adjusting the pH in the washwater with carbon dioxide. The last mentioned washing process isdescribed in EP 0 296 198 (AGA).

Thus, carbonates and carbon dioxide in the wash water are known toinfluence the extractives content of the washed pulp. However, there isno suggestion of using carbonates for deresination in the alkalinebleaching or extraction stages preceding such a washing procedure.

There are a number of alkaline reaction stages which are separate fromsaid washing procedures and which are typically included in theprocessing of pulps in a pulp mill. Such alkaline processes includeoxygen delignification stages, peroxide bleaching stages and extractionstages. In said stages, the pH is typically raised for the reaction inquestion to above pH 10 with the use of an alkali such as sodiumhydroxide or oxidized white liquor. It is also known to use otheralkaline agents such as sodium carbonate in this connection. Althoughsodium carbonate is not preferred as an alkaline agent, it is a muchcheaper industrial chemical than sodium hydroxide and is thereforesometimes selected for providing a high pH.

WO 00/47813 (STFI) describes a process for oxygen delignification of achemical pulp, wherein half of the alkali consists of carbonates and thepH is adjusted to exclude bicarbonates. The carbonates are used fortheir big buffering capacity, which gives an even alkali profile duringthe process. The Kappa number of the pulp is reduced by the process.

SE 219 677 (Watanabe) describes a process for the purification of pulpby oxidation of a pulp with oxygen or air. The purification is startedby soaking the pulp in a solution of sodium hydroxide or sodiumcarbonate. Thereafter the pulp is compressed, disintegrated and placedin water. Air or oxygen is applied at an elevated temperature and thenthe oxidized pulp is washed with water. The oxidation removes resins andprovides a light pulp which is easy to bleach.

U.S. Pat. No. 3,055,794 describes a method for the deresination of asulfite pulp in connection with the removal of the cooking lye and thewashing of the pulp. The acidic pulp is washed with one or two alkalinewashing mediums including sodium hydroxide and/or soda, and finally withwater to extract resins without mechanical action. The resin extractedpulp may be acidified directly after the washing operation.

SE 522 161 (Kvaerner) discloses an oxygen delignification processwherein sodium hydroxide is added to the reactor in connection with thegeneration of carbon dioxide therein. The subsequent formation ofcarbonate and bicarbonate ions provide a buffering action. An improvedselective Kappa reduction is obtained.

U.S. Pat. No. 4,445,969 (Samuelson) describes a three stage bleachingstage wherein alkali is supplied in the form of carbonate and liberatedcarbon dioxide gas is removed.

Manoucheri and Samuelson, Oxygen bleaching of kraft pulp, Svenskpapperstidning, 12, 1975, p 451 to 454, describe experiments usingcarbonates and bicarbonates as the added alkali. An increased carbondioxide concentration is said to retard the delignification.

EP 1 266 994 (Weyerhauser) describes bleaching of mechanical pulps withhydrogen peroxide and using magnesium hydroxide and sodium carbonate asalkali. The brightness of the pulp is increased by the process.

Thus, the prior art includes the use of carbonates in the alkalinebleaching of pulps and notes its influence on the brightness and theKappa number. The Kappa number is a widely used measure of the lignincontent of a pulp. There is no suggestion in the prior art, however,that the carbonates in the alkaline stages might improve thederesination of the pulp. The presence of carbon dioxide in an oxygendelignification stage is rather considered to impair the reaction.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a process forimproving the deresination of a pulp. The objects of the invention areachieved by a process and use, which are characterized by what is statedin the independent claims. The preferred embodiments of the inventionare disclosed in the dependent claims.

The invention is based on the realization that carbonate ions can beactively utilized for improving the deresination of resinous pulps whenthe pulp is being treated in a strongly alkaline process stage,preferably at an elevated temperature. It has been realized that in analkaline process stage the conditions are more favourable for thesolubilization of the wood extractives than in a washing stage. Providedthat the pulp is dewatered after the alkaline process stage underconditions, which maintain the solubilized extractives in solution,improved deresination of the pulp is obtained.

Accordingly, the present invention in a first aspect provides a processfor the deresination of pulp, which comprises providing an aqueous pulpsuspension, which contains wood extractives in water-insoluble form;subjecting said pulp suspension to an alkaline process stage other thana washing stage at a pH above 10; in said alkaline stage, subjectingsaid pulp suspension to the action of carbonate ions for solubilizing asignificant part of said wood extractives; said carbonate ions beingprovided by adding a sufficient amount of a substance selected fromcarbonate compounds and carbonate-forming substances for causing saidsolubilization; and dewatering the resulting carbonate-treated pulpsuspension without lowering the pH below 10 for removing solubilizedextractives from said pulp and for providing a deresinated pulp.

In the preferred embodiment of the invention, the carbonate ions areprovided by a carbonate forming substance, which comprises carbondioxide. Most preferably the carbonate forming substance is provided inthe form of gaseous carbon dioxide or a gas, wherein the major componentis carbon dioxide. The carbon dioxide is added to the pulp so thatcarbonate ions are present in the alkaline process. The carbon dioxideis preferably added into the alkaline process stage itself or to a pipeline leading to the alkaline process stage. The carbon dioxide may alsobe combined with a gaseous or vapour phase component which is used forthe alkaline process stage.

When carbon dioxide is used to provide the carbonate ions, the amount ofcarbon dioxide added may be such that it has a pH lowering effect on theaqueous pulp suspension. In such a case, it may be necessary to counterthe pH-lowering effect by adding extra alkali to the alkaline processstage to maintain the pH at a desired level above pH 10.

In another embodiment the carbonate compound comprises an aqueoussolution of an alkali metal (bi)carbonate.

In one embodiment of the invention, the carbonate compounds are providedby liquors from other parts of the mill which contain carbonate and/orbicarbonate compounds, e.g. bicarbonate brine from a tall oil recoveryprocess or liquors from black liquor gasification or gas scrubbingliquors.

In a further embodiment of the invention, a (bi)carbonate solutioncontaining a desired level of (bi)carbonates and a desired pH may beproduced by reaction of suitable acids and bases outside the mainprocess. An apparatus suitable for such a reaction is described in EP 1461 499 (Linde).

When liquors comprising bicarbonate compounds are used to provide thecarbonate ions, the amount of liquid added may be such that it has a pHlowering effect on the aqueous pulp suspension. In such a case, it maybe necessary to counter the pH-lowering effect by adding extra alkali tothe alkaline process stage to maintain the pH at a desired level abovepH 10.

The pulp suspension in the alkaline process stage in question mayalready contain some carbonate ions deriving from other operations inthe mill. The carbonates may, for instance be a result of the use ofsodium carbonate as an alkali or the carbonates may derive from carbondioxide used for improving the washing of the pulp in a preceding orsubsequent process stage. Some carbonate may also derive from thecooking of the wood fibres. In order for the carbonate ions to performtheir deresinating action according to the invention, the carbonatecontent should be controlled to between 30 and 200 mmol/l of the aqueousphase of the pulp. According to an embodiment of the invention, the pulpto be treated has an initial carbonate ion content and the carbonate ionconcentration in the pulp increased by the addition of carbon dioxide tosaid pulp.

One aspect of the invention relates to the use of carbon dioxide or tothe use of a liquid containing carbonate and/or to the use ofbicarbonate brine for the deresination of an aqueous pulp suspension inan alkaline process stage operating at a pH above 10.5. The deresinatedpulp so obtained is preferably further processed into a dried productselected from paper, board, paperboard and dry pulp in a manner knownper se.

DESCRIPTION OF THE INVENTION

According to the invention an aqueous pulp suspension is deresinatedwith the aid of carbonate ions. The pulp suspension contains woodextractives in water-insoluble form and the extractives are solubilizedby the action of the carbonate ions. The carbonate ions are caused toact on the pulp in a strongly alkaline environment so that insolublecompounds are solubilized and can be removed in the subsequentdewatering stage.

In the present specification and claims, the term “deresination” meansthe removal of wood extractives from the aqueous pulp suspension whichis being treated. The deresination obtained by the present inventionshould be significant in the sense that it should bring about a clearlynoticeable improvement in the process in the form of reduced clogging,reduced dirt specks in the product and less problems related to thepresence of extractives such as bad smell and taste. The improvementshould be understood as referring to a comparison to an otherwisesimilar process but lacking the carbonate levels of the presentinvention. The content of certain extractives is reduced by at least 25%and preferably at least 30% by the process of the invention. Thisspecific reduction refers primarily to the unsaturated fatty acidsincluded as part of the extractives.

In the present specification and claims, the terms “solubilizing” or“solubilized” is used to indicate that extractive components which areinsoluble or sparingly soluble in the aqueous phase of the pulpsuspension is made to enter the aqueous phase in a form whichfacilitates the removal of extractives with the removal of said theaqueous phase. Especially, the unsaturated fatty acids are componentswhich may be dissolved or dissociated and they preferably form micelles.

In the present specification and claims, the term “carbonate ion” refersto ions having the chemical formula CO₃ ²⁻. In the specification andclaims, the term “carbonate” is sometimes used to denote the carbonateion, such as in the terms “carbonate action” or “carbonate treatment”.The carbonate ion is an anion, which is more or less closely associatedwith a cation such as sodium Na⁺ or calcium Ca²⁺. The term “carbonatecompound” refers to a chemical compound such as sodium carbonate, sodiumbicarbonate, calcium carbonate, etc. The carbonate compounds of theinvention are capable of dissociating in an aqueous medium and ofproviding carbonate ions therein.

In the present specification and claims, the term “carbonate-formingsubstance” refers to a compound or mixture, which is capable of creatingcarbonate ions in the conditions of the process in question. Thepreferred carbonate-forming substance is carbon dioxide, which formscarbonate ions in water according the equation

CO₂+H₂O→H₂CO₃→2H⁺+CO₃ ²⁻

The present invention is suitable for the deresination of both chemicaland mechanical pulps. Both chemically and mechanically produced pulpsinclude wood extractives which may cause problems at some point of theprocess. During the handling of the fibres from the wooden raw materialto the end product, the fibres undergo various treatments, wherein woodextractives are liberated and may be either solubilized or desolubilizeddepending on the prevailing conditions. For instance, resin acids andfatty acids are relatively soluble when they are in the form of sodiumsoaps while the corresponding calcium soaps are fairly insoluble andwill stick to fibres and equipment. Under favourable conditions thesolubilized soaps, especially the soaps of the unsaturated fatty acids,form inmicelles, which in turn are capable of picking up andsolubilizing also such species, which are not in themselves soluble. Themicelles are broken and the solubilized compounds are deposited on solidsurfaces such as inner walls of pumps and pipes, in washers, and onfibers, if the conditions for micelle formation are disturbed.

It has been found that the solubility of the wood extractives, andespecially the solubility of the unsaturated fatty acid soaps of theextractives is significantly increased by the presence of carbonate ionsin a strongly alkaline environment. This phenomenon is utilized in thepresent invention by providing a sufficient carbonate ion content in analkaline process step to solubilize extractives. By removing thesolubilized components under conditions, which favour the maintaining ofthe micelles in intact form, a significant improvement in thederesination is obtained.

Since the greatest problems with extractives are encountered when thepulp is of hardwood origin or contains hardwood pulp, the greatestbenefits of the invention are provided in a process, wherein the woodextractives comprise extractives from hardwood. Under favourableconditions, also betulinol, which is an especially problematic componentof birch extractives, can be removed.

The pulp, which is to be subjected to the deresination process of theinvention may be a chemical pulp, a mechanical pulp, a semi-mechanicalpulp, a totally or partly recycled pulp or a mixture of such pulps. Thepulp to be treated may, for instance, be a kraft pulp, sulfite pulp,groundwood pulp, thermomechanical pulp, chemithermomechanical pulp,high-yield pulp, deinked recovered paper pulp, stone groundwood, refinermechanical pulp. In a preferred embodiment, the pulp contains kraft pulpof hardwood, softwood or mixtures thereof.

The influence of carbonate ions on the solubility of the extractives haspreviously been utilized in the washing of pulps, especially after theoxygen delignification or hydrogen peroxide bleaching stages. However,in the present invention, carbonate ions are made to act on the pulp inthe alkaline stage itself. In this manner, the conditions forsolubilization can be adjusted so as to greatly improve the deresinationcompared to performing a washing operation with carbonate-rich water.

The alkaline process stage of the invention should provide asufficiently high pH, i.e. one above pH 10. The pH is preferably 10.5 orhigher, especially between pH 11 and 14. The alkaline process step ispreferably operated at a pH of 11 to 13.5. The pH value in thespecification and claims refers to a value measured at room temperature(about 25° C.). Suitable alkaline process stages comprise oxygendelignification stages, peroxide bleaching stages and extraction stages.Such stages are typically designated O, P and E, respectively, inchemical pulping.

In a preferred embodiment of the invention, the alkaline process stagecomprises an oxygen delignification stage and the pH of the aqueous pulpsuspension during the oxygen delignification stage is maintained at avalue above 10, preferably pH 11 to 13.5, by an alkali. The alkali usedin the oxygen delignification stage is typically sodium hydroxide oroxidized white liquor, the main component of which is sodium hydroxide.If desired, the alkali may also include sodium carbonate or bicarbonate.Mixtures of alkalis may also be used. Various other chemicals known tothose skilled in the art may also be added to the oxygen delignificationstage provided that they do not impair the action of the carbonate ionson the deresination.

In a preferred embodiment of the invention the carbonate formingsubstance comprises carbon dioxide. The carbon dioxide is preferablygaseous carbon dioxide or a gas containing carbon dioxide. It is,possible e.g. in an oxygen delignification stage to mix the carbondioxide with the oxygen gas prior to feeding it into the reactionchamber. The carbon dioxide may also be added to steam which is used forheating the pulp prior to the alkaline process stage. However, it ispreferable to add the carbon dioxide separately and controllably toprovide a desired concentration of carbonate ions. The carbonateconcentration may be analysed by conventional means. When the carbondioxide is fed into the pulp, it is preferably fed into the pulp in thealkaline process stage itself or prior to said stage, such as into apipe leading into the process stage. When gaseous carbon dioxide or agas containing carbon dioxide is fed into a pipe line, it is preferablyfed to the pressure side of a pump. The carbon dioxide may also be mixedinto water used for diluting the pulp suspension in or before thealkaline process stage. Carbon dioxide may be fed also to alkalinestorage stages or alkaline transport stages such as storage towers andstand pipes.

When the carbonate ions are provided by carbon dioxide, which has a pHlowering effect on the aqueous pulp suspension, this pH-lowering effectis preferably countered by adding extra alkali to the process stage tomaintain the pH at a desired level above pH 10. Also bicarbonates have atendency to lower the pH of the alkaline pulp suspension and the pH mayneed to be adjusted by using more alkali if bicarbonate is used toprovide the carbonate ions.

In another embodiment of the invention, the carbonate compounds of theinvention are provided by liquor of the mill. Such liquors compriserecover liquids comprising carbonate and/or bicarbonate compounds, e.g.brine from tall oil recovery processes, wherein carbon dioxide has beenused for acidulation or liquors from black liquor gasification or gasscrubbing liquors. It is possible to feed the recover liquid into thepulp in the alkaline process stage itself or prior to said stage such asinto a pipe leading into the process stage. The recover liquids may alsobe mixed into water used for diluting the pulp suspension in or beforethe alkaline process stage.

The pulp suspension entering the alkaline stage may initially includecarbonate ions deriving from some other process step. In the preferredembodiment of the invention, carbon dioxide is added to the pulpsuspension for increasing the carbonate ion concentration in the aqueousphase of the pulp suspension. According to the invention, a carbonatecompound or carbonate-forming compound is added to the pulp in an amountsufficient for providing a carbonate ion concentration of 30 to 200mmol/l, preferably 40 to 140 mmol/l and most preferably 60 to 120 mmol/lin the aqueous phase of the pulp suspension.

Adding solid carbonate compounds such as sodium carbonate or sodiumbicarbonate into a continuous industrial process, such as a pulpingprocess, is technically possible but it is also rather cumbersome andmessy. The solid compound needs to be weighed and dissolved in waterbefore the addition and this requires a lot of handling and manual work.Moreover, when carbonates are added into the process in the form ofsodium carbonate or sodium bicarbonate, the chemical at the same timeadds extra sodium. That disturbs the sodium/sulphur balance in theclosed part of the mill.

In accordance with the preferred embodiment of the present invention thecarbonate ions are provided by carbon dioxide which is easy to handle inthe mill and which does not add sodium or any other component to theprocess. The carbon dioxide reacts with the water and provides thedesired carbonates in a simple and elegant manner.

In the prior art, there has been suggested that carbon dioxide in analkaline reaction stage such as the oxygen delignification stage, mightbe harmful for the delignification. However, in accordance with thepresent invention, the carbon dioxide fed into the pulp suspension hasnot been found to cause any harm. The carbon dioxide is quicklydissolved in the alkaline aqueous phase of the suspension and anypH-lowering effect is countered by additional alkali. The amount ofcarbon dioxide fed into the pulp is typically from 0.5 to 10 kg per tonbone dry cellulose, preferably from 1 to 7 kg per ton and mostpreferably from 2 to 4 kg per ton. The amount of carbon dioxide neededmay be calculated by measuring the carbonate concentration or bymeasuring the surface tension of the pulp suspension.

By adding 10 kg of carbon dioxide to a pulp stream for an oxygendelignification stage, the carbonate content will be immediatelyincreased by about 31 mmol/l. When the recirculation of filtrate hasreached equilibrium, the carbonate content will have increased by 68mmol/l from the initial carbonate level of the pulp.

When a (bi)carbonate solution or a liquor from other parts of the millis used, the amounts of carbonate ions can be easily analysed and/orcalculated to provide a corresponding increase in carbonate content.

A sodium bicarbonate brine from a carbon dioxide based tall oil recoveryprocess will typically contain carbon dioxide about 5-30 kg/m³. Theamount of bicarbonate brine liquid fed into the pulp is typically from0.01 m³ to 1 m³ per ton bone dry cellulose, preferably 0.05 to 0.80 m³per ton and most preferably from 0.15 m³ to 0.60 m³ per ton.

The tall oil recovery brine includes in addition to carbon dioxide and(bi)carbonate, also water soluble extractives, which may assist in thederesination.

In addition to pH, temperature is a factor which influences thesolubilization of the wood extractives and an elevated temperature ispreferred. The alkaline process stage is preferably operated at atemperature between 60 and 130° C., more preferably at 75 to 110° C.

A prolonged exposure to the action of the carbonate ions improves themicelle formation and hence enhances the deresination of the pulpsuspension. In the traditional washing of bleached pulp withcarbonate-rich water, the exposure time is very short, typically amatter of seconds. In the preferred embodiment of the present invention,the pulp suspension is subjected to the carbonate action for a longertime. The exposure time should be at least 5 minutes and preferably 10minutes, or more. The exposure time is preferably between 0.5 and 3hours, more preferably between 1 and 2 hours.

Without wishing to be bound by any theory, it is believed that thebeneficial action of the carbonate ions is at least partly dependent onthe action of the carbonate ions on sparingly soluble calcium soaps ofunsaturated fatty acids such as oleic acid, linoleic acid and linolenicacid. At a high pH (above pH 10 and especially above pH 10.5) thecarbonate ions cause said soaps to form solubilized free anions andthese, in turn, form micelles capable of capturing undissolvedextractive components. This facilitates the removal of both solubilizedsoaps and insoluble extractive components in a subsequent dewateringstep. If the pH is lowered below about 10, the micelles start todisintegrate and the extractives are again deposited on the fibres orthe equipment.

In a traditional pulping process there are certain steps such as one ormore chlorine dioxide bleaching steps, which affect the unsaturatedfatty acids and tend to saturate them. The saturated fatty acids do notseem to have the same capacity to form micelles as the unsaturated ones.Therefore, the pulp suspension should preferably be subjected to thecarbonate action in the alkaline process stage prior to being subjectedto any stage capable of significantly saturating the unsaturated fattyacids of the wood extractives.

When the pulp suspension in the alkaline process step contains asufficient amount of free unsaturated fatty acids, the surface tensionof the aqueous phase is at a substantially constant level. This level ispreferably below 35 mN/m and more preferably below 30 mN/m measured at70° C. according to the well known Du Noüy Ring method.

When the micelles start to disintegrate, the surface tension rises.Therefore, the dewatering stage should not only retain a high pH but itshould preferably also retain a substantially constant surface tension,preferably below 35 mN/m. When the filtrate of the pulp is analysed, thesurface tension should preferably not change more than 5% when the pulpis diluted with equal amounts of deionized water (1:1). When the pulpsuspension in the alkaline process stage has been treated with carbonateions in the desired amount, the system becomes robust and can be dilutedas much as 1:1 without reaching the critical micelle concentration atwhich the micelles disintegrate. The surface tension is an importantfactor since it indicates the amount of washing water that can safely beintroduced into the pulp for the dewatering stage without destroying themicelles.

The dewatering of the carbonate treated alkaline pulp may be performedin any standard manner providing that the pulp is not diluted with suchan amount of water that the pH falls below the critical value of 10.Preferably the pulp has a pH of at least 10.5 and more preferably pH 11or higher, when being dewatered.

The dewatering of the carbonate-treated pulp provides a deresinatedpulp, which may be used in a well known manner. The deresinated pulp ispreferably further processed into an end product. The end product istypically a dried product selected from paper, board, paperboard and drypulp. The procedures for processing a dewatered and deresinated pulpinto such end products are well known to those skilled in the art andsuch procedures are therefore not described in detail herein.

In a preferred embodiment of the invention, a kraft pulp suspensioncontaining pulp selected from softwood pulp, hardwood pulp and mixturesthereof is subjected to an oxygen delignification stage at a pH of 11 to13 provided by the introduction of oxidized white liquor into said pulp,and gaseous carbon dioxide is injected into said pulp suspension in anamount of 0.5 to 10 kg per ton bone dry cellulose to increase thecarbonate ion content of the suspension and improve the solubilizationof the wood extractives, whereafter the delignified pulp suspension isdewatered at a pH of no less than 11.

The above description illustrates the use of carbonate compounds andcarbonate-forming substances for the deresination of an aqueous pulpsuspension in an alkaline process stage operating at a pH above 10. Inthe preferred embodiment, the carbonate ions are provided by gaseouscarbon dioxide.

The following example illustrates the invention.

Example

Reference Process

A kraft pulp suspension made of hardwood pulp is subjected to oxygendelignification. The carbonate content of the pulp is analysed and isfound to be about 25 mmol/l. The pulp is first mixed with oxidized whiteliquor to raise the pH for the oxygen delignification. Then oxygen isadded to the pulp and it is fed into the delignification reactor. Thetemperature of the reactor is raised to about 95° C. and the reaction isallowed to proceed for about 70 minutes.

The pH at the end of the oxygen delignification reaction is about 11.5.The pulp is diluted with filtrate from a washer and dewatered. The pulpso obtained is washed, treated further in a bleaching line, washedagain, stored and is then fed to a paper machine to produce paper. Thedewatering apparatus after the oxygen delignification stage is badlyclogged by extractives coming from the delignified pulp. Also the laterwashing stages suffer from sticky deposits. The paper produced in thepaper machine contains a significant amount of darks specks due to theextractives in the pulp.

Process of the Invention

The same delignification operation as above is repeated with thedifference that carbon dioxide is fed into the pulp after the alkalifeed and before the oxygen feed. The amount of carbon dioxide is 8 kgper bone dry ton of pulp, which raises the carbonate content of theaqueous phase of the pulp to 79 mmol/l. The alkali feed is increased tocompensate for the pH-lowering effect of the carbon dioxide.

At the end of the delignification reaction the pH is 11.5 and in thedewatering the pH remains at a value above 11. The dewatered pulp istreated as above and used to make paper.

Due to the action of carbonate ions on the extractives in the highlyalkaline reaction stage, a significant portion of the extractives areremoved with the washing waters. There is significantly less clogging ofthe dewatering equipment after the oxygen delignification stage and thepaper produced has an improved quality and fewer dark specks.

1. A process for the deresination of pulp, characterized in providing anaqueous pulp suspension, which contains wood extractives inwater-insoluble form; subjecting said pulp suspension to an alkalineprocess stage other than a washing stage at a pH above 10; in saidalkaline stage, subjecting said pulp suspension to the action ofcarbonate ions for solubilizing a significant part of said woodextractives; said carbonate ions being provided by adding a sufficientamount of a substance selected from carbonate compounds andcarbonate-forming substances for causing said solubilization; anddewatering the resulting carbonate-treated pulp suspension withoutlowering the pH below 10 for removing solubilized extractives from saidpulp and for providing a deresinated pulp.
 2. A process according toclaim 1, wherein said wood extractives comprise extractives fromhardwood.
 3. A process according to claim 1, wherein said pulp isselected from chemical pulps, mechanical pulps, semi-mechanical pulps,recycled pulps and mixtures thereof.
 4. A process according to claim 3,wherein said pulp contains kraft pulp of hardwood, softwood or mixturesthereof.
 5. A process according to claim 1, wherein said alkalineprocess stage is selected from an oxygen delignification stage, aperoxide bleaching stage and an extraction stage.
 6. A process accordingto claim 1, wherein said alkaline process stage comprises an oxygendelignification stage and the pH of said aqueous pulp suspension duringsaid process stage is maintained at a value above 10 by an alkaliselected from sodium hydroxide, sodium carbonate, oxidized white liquorand mixtures thereof.
 7. A process according to claim 1, wherein saidcarbonate forming substance comprises carbon dioxide or a liquidcontaining carbonate and/or bicarbonate ions.
 8. A process according toclaim 7, wherein said substance is gaseous carbon dioxide.
 9. A processaccording to claim 7, wherein said liquid containing carbonate and/orbicarbonate ions is produced by reaction of suitable acids and basesoutside the main process.
 10. A process according to claim 7, whereinsaid carbon dioxide or liquid containing carbonate and/or bicarbonate isadded to said alkaline process stage or to a pipe line leading to saidprocess stage.
 11. A process according to claim 7 wherein gaseous carbondioxide is combined with a gaseous or vapour phase component used forsaid process stage.
 12. A process according to claim 10, wherein gaseouscarbon dioxide is added in an amount which has a pH-lowering effect onthe aqueous pulp suspension and said pH-lowering effect is countered byadding extra alkali to said process stage to maintain the pH at adesired level above pH
 10. 13. A process according to claim 7, whereinsaid carbonate forming substance comprises sodium bicarbonate brine froma tall oil recovery process.
 14. A process according to claim 13,wherein sodium bicarbonate brine is added in an amount which has apH-lowering effect on the aqueous pulp suspension and said pH-loweringeffect is countered by adding extra alkali to said process stage tomaintain the pH at a desired level above pH
 10. 15. A process accordingto claim 1, wherein said carbonate compound or carbonate-formingcompound is added in an amount sufficient for providing a carbonate ionconcentration of 30 to 200 mmol/l in the aqueous phase of said pulpsuspension.
 16. A process according to claim 1, wherein said alkalineprocess stage is operated at a temperature between 60 and 130° C.
 17. Aprocess according to claim 1, wherein said pulp suspension is subjectedto said carbonate action for a time between 5 minutes and 3 hours.
 18. Aprocess according to claim 1, wherein said pulp suspension is subjectedto said carbonate action in said alkaline process stage prior to beingsubjected to any stage capable of significantly saturating theunsaturated fatty acids of said wood extractives.
 19. A processaccording to claim 1, wherein the surface tension of the aqueous phaseof said pulp suspension in said alkaline process stage is below 35 mN/nmeasured at 70° C. according to the Du Noüy Ring method in said processstage and in said dewatering.
 20. A process according to claim 19,wherein said surface tension is changed no more than 5% when the pulp isdiluted with equal amounts of deionized water (1:1).
 21. A processaccording to claim 1, wherein a kraft pulp suspension containing pulpselected from softwood pulp, hardwood pulp and mixtures thereof issubjected to an oxygen delignification stage at a pH of 11 to 13provided by the introduction of oxidized white liquor into said pulp,and gaseous carbon dioxide is injected into said pulp suspension in anamount of 0.5 to 10 kg per ton bone dry cellulose to increase thecarbonate ion content of the suspension and improve the solubilizationof the wood extractives, whereafter said delignified pulp suspension isdewatered at a pH of no less than
 11. 22. A process according to claim1, wherein a kraft pulp suspension containing pulp selected fromsoftwood pulp, hardwood pulp and mixtures thereof is subjected to anoxygen delignification stage at a pH of 11 to 13 provided by theintroduction of oxidized white liquor into said pulp, and sodiumbicarbonate brine is injected into said pulp suspension in an amount of0.01 m³ to 1 m³ per ton bone dry cellulose to increase the carbonate ioncontent of the suspension and improve the solubilization of the woodextractives, whereafter said delignified pulp suspension is dewatered ata pH of no less than
 11. 23. A process according to any one of thepreceding claims, wherein said deresinated pulp is further processedinto a dried product selected from paper, board, paperboard and drypulp.
 24. Use of carbon dioxide for the deresination of an aqueous pulpsuspension in an alkaline process stage operating at a pH above
 10. 25.Use of a liquid containing carbonate and/or bicarbonate ions forderesination of an aqueous pulp suspension in an alkaline process stageoperating at a pH above
 10. 26. The use according to claim 25 for thederesination of an aqueous pulp suspension in an alkaline process stageoperating at a pH above 10, wherein said bicarbonate is sodiumbicarbonate brine from a tall oil recovery process.